This invention relates to interferometric methods and apparatus for measuring the distance to a surface, particularly changes in the distance, and has one application in the measurement of surface profiles. It can also be applied to the measurement of surface displacements, velocities, accelerations and vibrations.
A form of interferometry has been disclosed in which a coherent light beam, eg from a laser, is split into two parts, one of which is given a frequency shift, from .omega. to .omega. + .delta..omega., and the other of which is reflected from the surface under investigation. The latter part experiences a frequency shift if the surface is moving parallel to the beam, or, more relevantly for profile measurement, a phase-shift depending on the distance to the surface. The frequency-shifted beam and the surface-reflected beam are added at a photosensitive detector having non-linear (suitably square-law) amplitude response, eg a photomultiplier, the phase of whose output at frequency .delta..omega. is a measure of the surface motion or profile. As applied to surface motion, this form of interferometry has been disclosed in "Applied Optics", Vol 10, No. 12, (December 1971) at pages 2722-2723 by Macovski et al, and in "Optics Communications", Vol 7, No. 3 (March 1973), pages 244-247, by Ohtsuka.
The above-summarised form has the common interferometric drawback that, for many purposes, it is too sensitive. This is because the phase of the signal at frequency .delta..omega. is given, for a change of distance or displacement parallel to the beam, by EQU (2.pi./.lambda.) . 2 . displacement,
Where .lambda. is the wavelength of the light and therefore very much smaller than the distance changes which are of engineering interest. Typically the latter may be of the order of one-thousandth of an inch. When measuring phase-changes, this large difference between the light wavelength and the distance changes of interest can inter alia result in ambiguities. The present invention employs a form of interferometry which alleviates this difficulty.